This invention pertains generally to high-acceleration propellants and particularly to support for these propellants.
High-acceleration rockets subject the propellant charge to severe mechanical stresses which can cause mechanical failure due to cracking, dewetting, unbonding, or permanent deformation. Rockets with the highest acceleration level are those launched from intermediate to large-caliber guns using post-lauched rockets or solid-fuel air-breathing propulsion systems to increase range, payload to a given range, or velocity at the target. These gunlaunched weapons are restricted envelope systems, for example, weight, diameter, length or combination of these variables are restricted by gun-system-imposed restraints. Mechanical failure of the propellant can also cause the propellant to crush the igniter into the exhaust nozzle, causing a detonation of the rocket.
Although maximizing the propulsion system volumetric performance in order to maximize the range, payload, or velocity improvement is the first objective, compromises are usually made in propellant energy, binder system, or other desired characteristics to minimize the mechanical damage to acceptable limits during a gun launch. An example of such a compromise might be to increase the binder content to achieve non-typical high strength at the expense of energy and combustion efficiency. Such compromises are undesirable since they impact adverserly on achievable system performance.
For a support structure to be effective in minimizing mechanical damages, it must have the strength and an almost continuous surface to contain the propellant, while it does not overly restrict the exhaust area of the rocket. One support currently being utilized is a perforated metal plate, but a propellant without a high mechanical strength extrudes through and clogs the perforations, causing a catastrophic failure of the rocket. A support, such as, the one described in U.S. Pat. No. 3,807,171 issued to Stanley Anderson, which is star-shaped and extends almost the entire length of the rocket casing, is effective only if the propellant is not too thick on the support. The flexible and frangible boot of the U.S. Pat. No. 3,609,977 issued to Joseph B. McCormack is primarily used to prevent a premature ignition of the rocket propellant before substantial completion of the initial booster phase. Resistance to mechanical failure arises primarily from the strength of the main propellant component.
Multi-component propellants have been used previously, usually one component is a high thrust propellant for lift off but numerous other uses have been served. From U.S. Pat. No. 3,858,392, issued to Evan et al. on Jan. 7, 1975, a first stage ignition is used to heat an adjacent second stage.
Polyurethane has often been used as a resin in composite propellants because of the crack resistance thereof, the adhesive property which allows the propellant to be bonded directly to the rocket chamber lining, rubbery mechanical qualities, low brittle point, excellent resilience and superior aging property. Many polyurethane propellants have been developed; examples are shown in U.S. Pat. No. 3,245,849, issued to Klager et al. on Apr. 12, 1966. Several thousand formulations are disclosed in this patent, including a formulation which comprises an aromatic di-isocyanate, and alkylene diol, a tri-functional crosslinker, an oxidizer, a plasticizer, and other additives such as anti-oxidents.